Abstract: In this study, we investigated the influence of hydrogen on the intergranular cracking and micro-deformation mechanism in an equiatomic CoCrNi medium entropy alloy (MEA) fabricated via laser powder bed fusion (LPBF). The LPBF-built CoCrNi MEA was rather sensitive to hydrogen embrittlement, whose elongation deteriorated significantly when introducing hydrogen concurrently during deformation. The fracture mode also changed from ductile dimples to brittle fracture facets containing tear-ridge patterns. According to the electron backscatter diffraction (EBSD) measurements, the preferential propagation path of these brittle cracks was detected along grain boundaries, i.e., intergranular (IG) fracture. In addition to the crack path, the IG crack morphological features and local plasticity evolutions were analyzed to characterize the hydrogen-induced IG cracking in LPBF-built material. More specifically, local grain refinement within the fusion boundaries resulted in a discontinuous IG crack morphology, including frequent crack coalescence and deflection, which should be responsible for forming tear-ridge patterns on IG fracture facets. Furthermore, the subgrain boundaries also significantly affected the IG cracking since they were made of dislocations, which could enrich local hydrogen concentration and promote IG crack propagation. As for micro-deformation mechanisms, hydrogen promoted the formation of stacking faults and deformation twins by reducing stacking fault energy. With ongoing hydrogen charging, the deformation twins were triggered along different planes simultaneously at the early stage of plastic deformation instead of exhibiting a sequential procedure from forming stacking faults, primary deformation twinning to secondary deformation twinning in the uncharged specimen.

Key words: Hydrogen embrittlement, CoCrNi medium entropy alloy, Additive manufacturing, Intergranular cracking, Deformation twinning